1. Field of the Invention
The present invention relates to a method and apparatus for placing a transducer over a selected track on a moving disc. More particularly, the invention relates to computation of a correction signal to improve the accuracy of a position demand signal for moving the transducer to a selected track.
2. Prior Art
The use of servo tracks, recorded on moving discs, recovered from the disc by a transducer, and having been recovered, being decoded to give indication of the positional deviation of the transducer from a central position, is well known as a method for finding specific locations or benchmarks on the moving disc, and thereby, reducing the positional errors encountered in the placing of a head over the disc, or in bringing the disc itself into some registration. The method has been used to position reading and writing transducers over other moving media also. U.S. Pat. No. 3,691,543 to Mueller shows such a system.
The particular physical embodiment of the servo track, employed in any particular device, depends on the application, and ranges from dedicated tracks, containing nothing except positional information, to signal storage tracks, having servo information interspersed among the stored signals. Another embodiment of a servo track is the use of the signal storage track itself to provide servo information where means are provided to interpret the signals.
The particular form of the readback signal from the servo track may also be one of many varieties. One example is the sophisticated, two-element so called "tribit" tracks as taught by Mueller in aforementioned U.S. Pat. No. 3,691,543.
All servo track systems have the commonality that their operation requires the provision, at the output of the transducer, of two, separable position indicating signals, retrieved from the moving media (e.g. disc). The signals are designed to show differences, from one another, whenever the transducer is not in the null position. The difference may be in amplitude, relative timing, or both. In every case, the reduction of the designated differences to zero is indicative of the transducer being at the central (or null) position of interaction with the servo track. To bring the transducer to this position, whether by moving the transducer or moving the disc, is to have found a benchmark on the disc. The precise knowledge of one positional point allows for similarly precise knowledge of the whereabouts of other points on the disc.
In order to use the servo information, derived from the servo track, it is necessary to provide a demodulator for interpreting the signals from the media, to provide indication of the displacement of the transducer from the null position. There is a different style of demodulator for every kind of servo track.
No matter what the specie of servo track, in every demodulating system the two, separable signals, derived from the media by the transducer, must be balanced in order to find the null position.
It is generally true that, in order to find the difference between any two quantities, it is necessary, in effect, to perform two measurements. The difference between the two measurements is then taken. The two measurements, which, generally, involve the production of a representation of each of the two quantities to be compared, may be made simultaneously, through two separate channels, or one after the other through the same channel.
Servo track demodulators operate, in nearly every case, by the simultaneous, rather than the time sequential measurement of both position indicating signals. Such simultaneous measurement of two quantities presents problems in the calibration, balance and relative offsets encountered between the two, nominally identical, measuring channels required to perform the comparison operation.
It is possible to achieve great precision in the location of the transducer's null position, on a servo track, by constructing demodulators of extreme accuracy in themselves. There are always problems however, in maintaining thermal, and lifetime stability in the two measuring channels. No matter how well the demodulator is first built, changes can and do appear with time and environment. Problems arise in the mutual calibration, balance and offsets between the two channels.
Other problems arise in the manufacture, on a large-scale basis, of such precision devices whereby component tolerances conspire to render the balance between the channels unacceptably poor, necessitate the inclusion, of manually adjustable elements for later set up and test.
The aforementioned Mueller patent has these deficiencies. The Mueller system contains a disc with a servo track, a transducer for picking up two position indication signals from the servo track, a servo for positioning the transducer, and a feedback circuit with a two channel demodulator for separating the two position indicating signals, comparing them, and providing a signal as the feedback signal to the servo. As the transducer approaches and crosses the servo track, the position indicating signals change these, changing the comparator signal which changes the servo signal.
It is therefore desirable to provide means whereby a positional measurement precision, of the servo track system, may be made largely independent of the precision of the demodulator. It is also desirable to provide means allowing for greater position finding precision in those systems employing conventional servo track demodulators. In particular, it is desirable to provide means whereby a system, employing a poorly balanced demodulator, may be made to function at least as well as a system employing a precision demodulator.